This disclosure relates to hydroxyaromatic-aldehyde resole resins, their preparation, use, and articles formed therefrom. Hydroxyaromatic-aldehyde resole resins, and in particular phenol-formaldehyde resole resins, are of utility in a wide range of applications due to their excellent physical properties, including their durability, water resistance, bond strength, and the like, as well as their low cost and ease of manufacture and use. Phenol-formaldehyde resole resins have accordingly been used in the manufacture of products as diverse as laminates, consolidated wood products, and fiberglass insulation materials.
While a wide variety of hydroxyaromatic-aldehyde resole resins have been developed and are suitable for their intended purposes, environmental and industry standards demand ever-increasing improvement in both environmental compliance and physical properties of the resins. Reduction in aldehyde (particularly formaldehyde) emissions has proved particularly difficult without significantly adversely affecting the advantageous properties of the resins, cost, and/or manufacturing time. For example, formaldehyde scavengers such as urea, ammonia, melamine, various primary and secondary amines, dicyandiamide, and other amino-based modifications have been added to resoles. These are typically post-added to the resin or at the customers' plant, resulting in low efficiencies. Post-addition of urea can cause trimethylamine odors, which arises from incomplete reaction of urea. Post-addition of ammonia as a scavenger can lead to lower water dilutability, unwanted precure, and ammonia odor.
Other approaches to reduction of formaldehyde emissions include post-addition of a cyclic urea prepolymer, as described in U.S. Pat. No. 6,114,491. This prepolymer is formed from and contains ammonia. A process of reacting a first amino-based scavenger under acidic conditions and a second amino-based scavenger at neutral or slightly basic conditions is described in U.S. Pat. No. 4,757,108. A process requiring adding ammonia, preferably at the site of the resin manufacturer, before the addition of urea, is described in U.S. Pat. No. 5,300,562.
There nonetheless remains a need in the art for improved compositions for use as binders for the manufacture of insulation, for example fiberglass insulation. Such binders are typically low molecular weight, phenol-formaldehyde resoles together with formaldehyde scavengers, acid catalysts, and coupling agents. Acid cure has been favored in the art because it produces a glass fiber insulation having good strength and moisture resistance characteristics. The most common scavengers are chemical species containing a primary or secondary amine functionality, for example urea, ammonia, melamine, and dicyandiamide. When urea is used as the formaldehyde scavenger, the amount of urea added to the resin is referred to as the extension level, which is reported as a percent of the binder solids. Binder solids consist of phenol-formaldehyde resole resin solids and extender solids.
The addition of formaldehyde scavengers to a phenol-formaldehyde resole resin requires a finite period of time to achieve equilibrium with the free formaldehyde in the resin. The process of reaching this equilibrium is referred to as pre-reaction, and the time to reach the equilibrium is referred to as the pre-react time. Pre-react times vary with temperature and amine species. When urea is used, the pre-react times range from 4 to 16 hours depending on temperature. Use of urea can also adversely affect the mole ratio of formaldehyde to urea in the binder, which is optimally maintained between 0.8 and 1.2. If the extension level results in a formaldehyde-urea ratio of less than 0.8, the opacity increases significantly along with the ammonia emissions. If the extension level results in a formaldehyde-urea ratio greater than 1.2, formaldehyde emissions increase and the risk of precipitation of dimethylolurea is greatly increased.
There are other disadvantages to pre-reacting resins with urea prior to forming the binder. Because free formaldehyde improves the solubility of phenol-formaldehyde tetradimer, or methylene bis-(4-(2,6-dimethylolphenol)), in the resin, pre-reacting with urea will reduce the percent of free formaldehyde in the resin, hence reducing resin tetradimer solubility over time. Further, long pre-react times, as observed when urea is used as the formaldehyde scavenger, shorten the shelf life of the binder.
There is accordingly a need for hydroxyaromatic-aldehyde resole resins and methods that will lower phenol and aldehyde (particularly formaldehyde) emissions from phenol-formaldehyde resole resins while maintaining or improving premix stability, cure efficiency, and/or advantageous physical properties such as moisture resistance, tensile strength, and compression recovery.